The present application claims priority to Korea Patent Application No. 10-2010-0093876 filed in the Republic of Korea on Sep. 28, 2010, the disclosures of which is hereby incorporated by reference in its entirety.
In a mobile communication system, a mobile terminal decides a serving base station (i.e., Serving eNodeB) thereof according to a fixed rule, registers in the serving base station, and then forms a wireless link to send or receive data to/from the other terminals. Since a mobile terminal moves around, the mobile terminal receives, from the serving base station, a signal having a received signal strength (RSS) varying accordingly to changes in position. For example, when a mobile terminal moves far away from the serving base station or enters into a shadow zone, the received signal strength of the signal received from the serving base station might decrease lower than a given threshold value RTH, making it difficult for the mobile terminal to send or receive data to/from the serving base station. In this case, the serving base station selects one of adjacent base stations and allows the mobile terminal to access the newly selected base station so that a call channel of the mobile terminal is continuously maintained. Such operation is commonly referred to as “handover”.
FIG. 1 is a diagram for illustrating a general handover process. In a common handover process, a source base station (i.e., source eNodeB) may denote a serving base station connected to a mobile terminal for a related service and a target base station (i.e., Target eNodeB) denotes a base station to be connected to the mobile terminal as a result of handover.
As shown in FIG. 1, when the mobile terminal (i.e., user equipment: UE) moves far away from the source base station, the received signal strength (RSS) RS of a signal received from the source base station decreases. In addition, when the RRS (RS) of the signal received from the source base station decreases lower than a given threshold value RTH, the source base station decides the execution of handover by selecting one of adjacent base stations as a target base station and allows the mobile terminal to access the newly selected target base station (since the received signal strength (RSS) RT of a signal received from the target base station increases as the mobile terminal moves toward it). Accordingly, the mobile terminal may continuously maintain the call channel for communication.
The handover may be classified into a hard handover and a soft handover. The hard handover establishes a new link between a mobile terminal and a target base station after releases an existing link between the mobile terminal and a service base station. Unlike the hard handover, the soft handover retains the existing connection between the mobile terminal and the serving base station until a new link is completely established between the mobile terminal and the target base station. Hereinafter, a typical handover process will be described based on the hard handover.
FIG. 2 shows a signal flow for illustrating a typical handover process. As shown in FIG. 2, the typical handover process may include a handover preparation process, a handover execution process, and a handover completion process.
In the handover preparation process, a mobile terminal measures a wireless quality and sends a measurement report thereof to a source base station (Source eNB) at step S101. The source base station determines whether to execute handover based on the measurement report at step S103. For example, when the source base station, based on the measurement result, determines that the received signal strength of the mobile terminal decreases lower than a threshold value, the source base station initiates handover at step S103. Upon the initiation of the handover, the source base station transmits a handover request message to a target base station (Target eNB) at step S105. The target base station performs a handover admission control operation at step S107 and sends a handover request confirmation message to the source base station at step S109.
The handover execution process is performed when the target base station accepts the handover request from the source base station in the handover preparing process as described above. In the handover execution process, the source base station sends a handover command to the mobile terminal at step S111. The mobile terminal performs an accessing procedure in order to access the target base station at step S113. The source base station forwards downstream data to the target base station while the mobile terminal performs the accessing procedure at step S115.
After the completion of the above handover execution process, the handover completion process is performed. For example, when the mobile terminal successfully accesses the target base station, the mobile terminal sends a handover completion message to the target base station at step S117). In response to the handover completion message, the target base station sends a handover completion response message to the mobile terminal at step S119. In this process, a data path between the source base station and a core network is changed to a data path between the target base station and the core network is established.